An aircraft is typically supported by plural pressurized landing gear struts. Designs of landing gear incorporate moving components which absorb the impact force of landing. Moving components of an aircraft landing gear shock absorber are commonly vertical telescopic elements. An alternate type of landing gear incorporates a trailing arm design which forms a triangle shape, where the main supporting body of the landing gear is hinged with a trailing arm, and a typical telescopic shock absorber functions as the third side of the triangle. The telescopic shock absorber of both types of landing gear comprise internal fluids, both hydraulic fluid and compressed nitrogen gas, and function to absorb the vertical descent forces generated when the aircraft lands.
Prior to a landing event, as the aircraft descends towards the runway, the landing gear is deployed. Each of the landing gear maintains an internal pre-charge pressure within the telescopic shock strut. The pre-charge pressure is a relatively low pressure, which is maintained to insure the landing gear shock absorber component is extended to full telescopic extension, prior to the aircraft landing. At full telescopic extension, the shock absorber can absorb its maximum amount of landing force. As the landing gear comes into initial contact with the ground, the strut begins to compress, thereby increasing the pressure within the shock absorber. Increases in pressure, beyond the pre-charge pressure, creates additional resistance to the compression rate of the landing gear strut, which helps reduce the vertical velocity of the aircraft.
The amount of force generated when an aircraft lands is a function of the aircraft weight at landing, and the vertical velocity at which that aircraft landing weight comes into initial contact with the ground. Aircraft have limitations regarding the maximum allowable force the aircraft landing gear and other supporting structures of the aircraft can safely absorb when the aircraft lands. Landing force limitations, which are often related to aircraft vertical velocity (sink-rate or sink-speed) at initial contact with the ground, are a key factor in determining the Maximum Landing Weight (“MLW”) for aircraft. The MLW limitation is related to an assumed aircraft Vertical Velocity at initial contact with the Ground (“hereinafter referred to as: VVG”).
Aircraft routinely depart from an airport with the aircraft weight less than the maximum take-off weight limitation, but greater than the maximum landing weight limitation. During the flight, in-route fuel is burned to reduce the aircraft weight, below the maximum landing weight limitation. On average, passenger airlines dispatch about 28,537 flights per day. With this high volume of daily flights, situations often arise where an aircraft has left the departure airport, and the pilot discovers the need to immediately return and land, without the time or opportunity to burn-off the planned in-route fuel. This causes an overweight landing event. When an overweight landing occurs, the Federal Aviation Administration (hereinafter referred to as “FAA”) in accordance with the aircraft manufacturer recommendations, require the aircraft be removed from service and a manual inspection be performed to check for damage of the landing gear and the connection fittings of the landing gear to the aircraft.
Because an overweight landing causes the aircraft to be removed from service for inspection, airlines work to avoid such events. As a result, an aircraft may take off with un-used capacity, because its take-off weight is less than the take-off weight limit. The weight carrying ability of the aircraft is thus limited, not by the maximum take-off weight limitation, but by the landing weight limitation.
The landing weight is limited by FAA Regulation. In studying the regulation and its history, an important realization was made.
The FAA is the Regulatory Authority which regulates the design, development, manufacture, modification and operation of all aircraft operated within the United States, and will be used along with the term “Regulatory Authority” to indicate both the FAA and/or any governmental organization (or designated entity) charged with the responsibility for either initial certification of aircraft or modifications to the certification. Examples of Regulatory Authorities would include: European Aviation Safety Agency “EASA”, within most European countries; Transport Canada, Civil Aviation Directorate “TCCA”, in Canada; Agencia Nacional de Aviayao Civil “ANAC” in Brazil; or other such respective Regulatory Authority within other such respective country.
FAA Regulations (provided in the Code of Federal Regulations) are the governmental regulations which detail the requirements necessary for an aircraft to receive certification by the Regulatory Authority within the United States. These would be equivalent to such regulations within the Joint Aviation Regulations “JARs” which are used in many European countries.
Title 14 of the Code of Federal Regulations, Part 25 refers to regulations which control the certification of Air Transport Category aircraft “Part 25 aircraft”. Part 25 aircraft include most of the commercial passenger aircraft in use today. For example, Part 25 aircraft includes Boeing model numbers 737, 747, 757, 767, 777; Airbus A300, A310, A320, A330, A340, etc.
In particular §25.473(a) provides:                Title 14—Code of Federal Regulations:        Part 25—Airworthiness Standards: Transport Category Airplanes        §25.473 Landing load conditions and assumptions.        (a) For the landing conditions specified in §25.479 to §25.485 the airplane is assumed to contact the ground—        (1) In the attitudes defined in §25.479 and §25.481;        (2) With a limit descent velocity of 10 fps at the design landing weight (the maximum weight for landing conditions at maximum descent velocity); and        (3) With a limit descent velocity of 6 fps at the design take-off weight (the maximum weight for landing conditions at a reduced descent velocity).        (4) The prescribed descent velocities may be modified if it is shown that the airplane has design features that make it impossible to develop these velocities.                    (emphasis provided: as to the underlined text)                        
Values described in the regulation and also herein, as they relate to vertical landing velocity, are expressed in feet per second “fps” and while these are typical values, they are used herein for example purposes only and are not limiting or restricting to the processes and methods described.
Chapter §25.473(a) of the FAA Regulations define the assumption as to the “limit descent velocity” which is the maximum VVG an aircraft is assumed to experience, during a landing event. This assumption is 10 feet per second “10 fps”.
The current rule evolved in the early Civil Aeronautics Board—Civil Air Regulations, Part 4b, §4b.230 (b) dating back to Nov. 9, 1945. The full text is provided herein as reference material, but the relevant text is in the final two paragraphs (i) and (ii).